A promising summer holiday is nearly ruined by the unfortunate arrival of Nancy
and Peggy Blackett's dreaded Great Aunt Maria (pronounced Mariah, no
doubt). Captain John then further upsets matters by damaging Swallow.
With both boats in dock, the Walkers take to camping on the western shore of the
Lake, on the edge of the moor, in a little valley they name Swallowdale.
Several lively adventures ensue as the Blackett sisters try to have a holiday in
spite of the presence of their domineering aunt. These include another encounter
with the charcoal-burners whom the children met in the same area the previous
year, and the daring ascent of Kanchenjunga - alias Coniston Old Man.

Also notable are a number of references to the childhood of Mrs Blackett and her
brother Jim 'Captain Flint' Turner, and allusions to the Peter Duck
mythology created by the children. Horrible Great Aunt Maria returns in The
Picts and the Martyrs, and the story of Peter Duck is told in the novel of the
same name.